Boston, Massachusetts Region LIDAR First Return Elevation Data, 2002

LIDAR (LIght Detection And Ranging) first return elevation data of the Boston, Massachusetts region from MassGIS at 1-meter resolution. This LIDAR data was captured in Spring 2002. LIDAR first return data (which shows the highest ground features, e.g. tree canopy, buildings etc.) can be used to produce a digital terrain model of the Earth's surface. This dataset consists of 74 First Return DEM tiles. The tiles are 4km by 4km areas corresponding with the MassGIS orthoimage index. This data set was collected using 3Di's Digital Airborne Topographic Imaging System II (DATIS II). The area of coverage corresponds to the following MassGIS orthophoto quads covering the Boston region (MassGIS orthophoto quad ID: 229890, 229894, 229898, 229902, 233886, 233890, 233894, 233898, 233902, 233906, 233910, 237890, 237894, 237898, 237902, 237906, 237910, 241890, 241894, 241898, 241902, 245898, 245902). The geographic extent of this dataset is the same as that of the MassGIS dataset: Boston, Massachusetts Region 1:5,000 Color Ortho Imagery (1/2-meter Resolution), 2001 and was used to produce the MassGIS dataset: Boston, Massachusetts, 2-Dimensional Building Footprints with Roof Height Data (from LIDAR data), 2002 [see cross references].

The data was collected for MassGIS to be used for a variety of purposes including emergency response planning, hydrologic / floodplain modeling, assessment of natural hazards, and impervious surface delineation.

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All materials, software, maps, studies, reports, and other products or data, regardless of physical form or characteristics, produced in furtherance of the Contract and funded, in whole or part, under the Contract shall be considered in the public domain and available to Executive Office of Environmental Affairs (EOEA) or its agencies at the reasonable cost of reproduction in any of the formats in which it is stored or maintained. The contractor shall not obtain, attempt to obtain or file for a patent, copyright, trademark or any other intrest in any such materials, software, maps, reports, and other products or data without the express, written consent of the EOEA and subject to any other approvals required by state and federal law.

Not Applicable Not Applicable The 3Di DATIS LIDAR system used multiple layers of system calibration to ensure horizontal accuracy. The initial instrument calibration was accomplished during the system fabrication. All critical components (IMU, LIDAR, Camera, and Scanner) were optically aligned to a common optical bench using NIST traceable optical collimators and optical flats. This alignment was initially carried out to 2 arc seconds. In addition to the optical bench calibration, the LIDAR system was operated against a series of known targets to determine both scale factor and bias accuracy. All system timing was derived from GPS. For aircraft installation the critical factor was location of the GPS antenna with respect to the DATIS optical reference. This was accomplished through a differential GPS survey. After installation in the aircraft, the system was then flown against a ground test range that was developed using differential GPS points from both static survey and a drive survey. This range provides several thousand points that were used to statistically verify the in flight calibration. Highest grade GPS equiptment was used to help ensure the horizontal accuracy. When flight conditions were acceptable and the predicted GPS PDOP (Position Dilution of Precision) factor was below 3.0 (with at least 5 GPS satellites and 6 desired) for the duration of the planned flight mission, the data acquisition mission was undertaken. The criteria for suitable flight conditions are clear visibility up to 2,000 m (6,560 ft. AGL (Above Ground Level)), calm to relatively low and steady wind speed with no icing conditions. Prior to takeoff the DATIS system IMU (Inertial Measurement Unit) was initialized on the ground for about 15 minutes. The plane (Cessna 206) then took off from Lawrence Municipal Airport, climbed to an altitude of 2,000 m and ferried to the project site. The DATIS system recorded data from the duration of the entire four hour mission and all laser, IMU, and GPS data were recorded onto 8mm tape cartridge. The field crew extracted all flight data from tape, downloaded the GPS receivers, post processed the GPS data, pre-processed the LIDAR data, and screened the collected flight data. This procedure was used to determine whether any area needed to be reflown the following day due to data gaps between flight lines or low quality GPS data. After all flight lines had been quality checked, the data was backed up onto 8mm cartridge tape and sent to 3Di's office in Boulder, Colorado, for post-processing and product generation.0.5meters RMSE Calibration of the system was verified each time two or more flight lines were combined. This was a result of taking measurements on a common point from multiple locations (i.e. the ground point is fixed and each flight line that sees this is a different instrument location). The calibration of the system was validated in terms of range and angular accuracies. Our standard practice requires that at least one flight line within the project be flown in both directions for calibration verification of range and angular accuracy. Another important area of quality control was edge matching. All edge matching discrepancies in LIDAR DEM data occur between two flight lines. The cause of this problem was nearly always associated with GPS data. Identification of edge matching discrepancies was performed by overlaying either the contour or the shaded relief model representation of the original DEMs. If edge artifacts were found in the overlap area between two flight lines, the GPS data was reevaluated to check for any possible errors in the post processing or a spike in the DOP at the time of data acquisition. Usually this type of problem was resolved by reprocessing the GPS data or in the worst case, reacquiring the data. For additional accuracy verification, static survey points were collected, using static benchmarks where available. Thirty-four survey points within the project boundary were selected to allow a statistical absolute elevation verification of the data. This data set was then statistically compared to the project LIDAR DEM data after the combination of flight lines to verify accuracy both horizontal and vertical. The RMSE (Root Mean Square Error) of the LIDAR DEM was calculated using the ground GPS data to ensure that the vertical error was less than 0.15 m. 0.14983 meters RMSE

Microsoft Windows XP Version 5.1 (Build 2600) Service Pack 2; ESRI ArcCatalog 9.2.2.1350.

Not applicable.